3. Towards the bottom of the Settings area, there's an advanced section where you have options to restore files, as well as contacts, calendars/reminders, and Safari bookmarks:

4. After clicking "Restore Files" a window will appear with a list of restorable files--simply check the box next to the files you wish to restore and click the restore button; the file(s) should be restored/accessible on your device again

Currently files are available for restoration 30 days after they're deleted (or marked as deleted). Files deleted beyond that 30 day window are not recoverable.